Problem: In chemistry class, the girl to boy ratio is $3$ to $8$. If there are $40$ boys, how many girls are there?
Answer: A ratio of $3$ girls to $8$ boys means that there is a group of $3$ girls for every group of $8$ boys. If there are $40$ boys, then there are $5$ groups of $8$ boys each. Because we know the ratio is $3:8$ , there must be $5$ groups of $3$ girls each. There is a total of $15$ girls in chemistry class.